Hydrogen sulfide (H.sub.2 S) is a toxic gas that is detectable by humans as a rotten egg odor at a concentration in air of 0.001 ppm or less. A human begins to lose ability to smell hydrogen sulfide at about 100 ppm thereof. Hydrogen sulfide becomes very toxic at about 800-1000 ppm accompanied sometimes with a brief sickeningly sweet smell. Human death occurs in 30 minutes when exposed to a concentration of 1000 ppm of hydrogen sulfide. The threshold limit value (TLV) imposed by OSHA for human exposure to hydrogen sulfide is 10 ppm for 8 hours, not to exceed 15 ppm for more than 15 minutes.
Serious hydrogen sulfide toxicity problems have arisen on ships employing a foaming agent mixed with water in fire fighting systems. Sulfate in seawater is chemically reduced by anaerobic bacteria present in the seawater to produce hydrogen sulfide in toxic amounts. Seawater contains about 800 ppm sulfur as sulfate. If all of the sulfate in seawater were converted to dissolved hydrogen sulfide, the concentration would be about 850 ppm, an amount which can be highly toxic to humans if released as a gas in air in a confined space.
The rate at which sulfate-reducing bacteria generate hydrogen sulfide from sulfate-containing waters depends on the activity of the bacteria which remove oxygen, on nutrients, temperature and other conditions. It may take several weeks or months to produce a dangerously high concentration of hydrogen sulfide in a fire fighting system. At that time, the system is normally flushed out and refilled with fresh foaming agent and seawater. This operation is expensive and may be damaging to the marine environment.
In crude oil, hydrogen sulfide is present in widely varying concentrations from zero to several hundred ppm, depending on the source of the crude oil. A knowledge of the concentration of hydrogen sulfide in crude oil, or in any commercially used liquid containing hydrogen sulfide, is desirable in dealing with possible problems of human toxicity arising during transportation and storage, environmental pollution, or subsequent refining operations.
Laboratory analysis procedures are available for determining hydrogen sulfide in liquids and gases and colorimetric test kits are on the market in the form of detector tubes and papers, badges and electronic instruments to test for hydrogen sulfide in the field. However, field test kits employing colorimetric indicators are rendered inoperable by samples producing foam on aeration which wets the indicator or by samples which are colored by dissolved, particulate or soluble matter. Field test kits for direct determination of hydrogen sulfide in crude oil and in fire-fighting foams are not available.